15- Nitrogen Fixation

Why is nitrogen important to plants and what is the biggest problem plants face with it?

• Nitrogen is a key element in plant nutrition

• Present in amino acids, proteins and nucleic acids

• Lithosphere (the mantle crust layer of earth) contains plenty of nitrogen but none of it is useful for plants.

• The atmosphere contains 80% N2 but the gas is inert.

What is nitrogen fixation?

• Nitrogen fixation is the process that converts inert (lacking the power or ability to move, act or react) atmospheric nitrogen gas into reactive, usable nitrogen compounds.

What kind of different methods of nitrogen fixation are there?

• Atmospheric Fixation, Industrial fixation and biological fixation

What is atmospheric fixation?

• Energy of lightning breaks nitrogen molecules apart and enables nitrogen atoms to combine with O2 forming N2O (nitrogen oxides)

• Nitrogen dissolve in rain, forming nitrates

• Nitrates are carried to the ground with the rain —> Nitrogen oxides dissolve in rain and changes into nitrates

What is industrial fixation?

• Under great pressure, at a temp of 600 degrees and the usage of a catalyst, atmospheric nitrogen (N2) and hydrogen are combined to form ammonia (NH3), which can then be used as a fertiliser.

What is biological nitrogen fixation?

• Many bacteria can fix nitrogen using an enzyme called nitrogenase. Process uses a lot of cellular energy. (ATP)

What is the nitrogen cycle?

• The process where nitrogen enters and leaves the biosphere through microbial action.

What are the key points of the nitrogen cycle?

• Fixation: by free living or symbiotic bacteria

• Ammonification: from organic to inorganic nitrogen

• Nitrification: from ammonia to nitrite and nitrite to nitrate

• Denitrification: from nitrite to nitrous oxide and nitrate to nitrogen gas

What are the different types of N fixers?

• Freeliving N fixers: Heterotrophic bacteria that get organic (from the environment and where N is limiting)

• Cyanobacteria (freeliving photo-autotrophs); symbiotic lichens (cyanobacteria with fungi offering physical protection)

What is ammonification?

• N is found in dead organic material (e.g. plants, animals, microorganisms) in the form of protein, DNA etc.

• Broken down by microorganisms to release ammonia

• Positively charged ammonium ions can bind to soil particles and also be taken by plants

What is nitrification?

• 2 step process where ammonium is converted to nitrite (nitrifying bacteria such as nitrosomas and nitrosococcus)

• Nitrite is converted to nitrate by Nitrobacter.- both nitrite and nitrate can be taken up by plants but are also easily leached by soil.

What is denitrification in the nitrogen cycle?

• Nitrate is converted into gaseous nitrogen by denitrifying bacteria

• Mainly occurs under low oxygen conditions

• Nitrate is used as a respiratory substrate by the bacteria

• Denitrifying bacteria live deep in the soil and in aquatic sediments where they use nitrates as an alternative to oxygen, leaving free nitrogen gas as a byproduct

What are the main differences between nitrites and nitrates?

Nitrites

• High levels in the soil are toxic to plants and microorganisms- chemically reactive, unstable, and quickly oxidise into nitrates or reduces into nitrogen gas.

• Reduces into nitrates/nitrogen gas.

Nitrates

• Readily absorbed by plant roots and vital for building proteins, nucleic acids, and driving rapid green vegetative growth.

What are N fixing in plants important to grow?

• Used in crop rotation

• Legumes provide protein to humans and other animals

• No group occupying more than half the land area at any time

How do rhizobium and plants communicate at the molecular level (give examples and draw a simple diagram)

• Rhizobium carries sym plasmid (legume flavanoids causes expression of nod genes encoded on sym plasmid)

• Nif and fix genes carried on sym plasmid encode enzymes to fix atmospheric nitrogen

• Some nod genes cause bacterium to produce chemical factors which induce plant to express plant nod genes and take up bacteria

Explain the communication between flavanoids and nod factors

1. Plant root produces specific flavanoids and nod factors

2. Most rhizobia produce nod factors, identifying them as appropraite symbionts

3. Plant preps to form a symbiotic nodule structure

Why do bacteroids need a high O2 flux but low O2 environment?

• Nitrogenase cannot be close to O2 but lots of ATP is required for oxidative phosphorylation

• Nitrogenase should not interact with O2 as O2 irreversibly damages and destroys the enzyme’s vital metal-sulfur cofactors

What is oxidative phosphorylation?

• Final stage of aerobic cellular respiration that uses energy released from the oxidation of nutrients to synthesize adenosine triphosphate (ATP).

What are the advantages and disadvantages of ‘‘green manure’’?

Advantages

• Adds organic matter by soil

• Increases biological activity

• Improves soil structure

• Increases the nutrients to soil

• Reduces leaching losses of nitrogen

• Suppresses weeds

• Reduces pest and disease problems

Disadvantages

• Cost of seed and extra cultivations

• Lost opportunities for cash cropping

• Extra work at the busiest times of year

• Exacerbated pest and disease problems (‘‘green bridge’’ effect)

• Potential for green manures to become needs themselves

Summarize what a root infection is

• Through flavanoids and nod factors plants and bacteria communicate with one another

• Bacteria attach to emerging root hairs, which begin to curl

• Bacteria multiply in curl an degrade plant cell wall

How does nod factor perception induce root hair curling?

• Nod factors are concentrated in the cell wall and are almost immobile

• Nod factors cause redirection of tip growth

• Only a few bacteria actually redirect the growth of the root hair successfully and become enveloped in an infection thread

Provide a summary of the plant Rhizobium interaction

• Bacteria multiply in vacuoles learned symbiosomes

• Found in cells of root nodules

• Plant leghaemoglobin provides bacteria respiration with O2 but buffers N reductase from excess O2 which is inhibitory

• Plant provides bacteria with carbohydrates

• Bacteria provide plant with ammonia

Why does a symbiotic nitrogen fixation require teamwork?

• Plants cannot fix nitrogen on their own and most rhizobia cannot fix nitrogen on their own

• Symbiotic nitrogen fixation is a true partnership, bacteria provide nitrogenase